Display device and method of driving the same

ABSTRACT

In a conventional EL display device, a general video signal (analog signal voltage) is sampled to be outputted to a source signal line and then inputted to a gate electrode of a driving TFT provided in each pixel. The drain current of the TFT is controlled to control the current flowing into an EL element, thereby adjusting the luminance. However, accurate gray scales cannot be obtained unless the video signal is converted before it is sampled and inputted to the source signal line since the drain current of the driving TFT does not correspond to the gate voltage of the driving TFT linearly. The invention solve this problem by converting an inputted analog signal voltage into a current, converting the current into the gate-source voltage of a correction TFT that has the same polarity as a driving TFT, supplying the gate-source voltage as a source line signal to a source signal line, and applying the source line signal to a gate electrode of the driving TFT. Thus an EL element can emit light at a luminance linearly corresponding to the inputted analog signal voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active matrix EL display device inwhich each pixel has TFTs (thin film transistors) and an EL element.Specifically, the invention relates to an active matrix EL displaydevice of analog gray scale system in which gray scales are reflectionof analog changes in amount of current flowing into EL elements.

In this specification, EL elements include those emit light from singletexcitation (fluorescence) and those emit light from triplet excitation(phosphorescence) both.

2. Description of the Related Art

With recent flood of data communication, demands for data communicationequipment are increasing. In data communication equipment, displaydevices for displaying images are indispensable. The display devicesthat are attracting attention are EL display devices using an EL elementthat is a self-luminous element.

As display units larger in size and higher in definition are needed indata communication equipment, active matrix display devices in whicheach pixel has TFTs are becoming the main stream display devices.

FIG. 4 is a block diagram of an active matrix EL display device. Asource signal line driving circuit 402 and a gate signal line drivingcircuit 403 are arranged in the periphery of a pixel portion 401. Asignal outputted from the source signal line driving circuit 402 isinputted to source signal lines S1 to Sx to be sent to pixels. A signaloutputted from the gate signal line driving circuit 403 is inputted togate signal lines G1 to Gy to be sent to pixels. Power supply lines(power lines) V1 to Vx are arranged in parallel to the source signallines to supply current to pixels.

As one way to reduce the size of a display device as well as manufacturecost, sometimes a pixel portion and a driving circuit portion (composedof a source signal line driving circuit and a gate signal line drivingcircuit) are formed on the same substrate. In this case, apolycrystalline semiconductor film is used to form TFTs that constitutethe pixel portion and the driving circuit portion.

This can be applied to the active matrix EL display device of FIG. 4,and an example of the pixel structure thereof is shown in FIG. 5.

A switching TFT 504 has a gate electrode connected to a gate signal lineG that is one of the gate signal lines G1 to Gy. The switching TFT alsohas a source region and a drain region one of which is connected to asource signal line S that is one of the source signal lines S1 to Sx andthe other of which is connected to one of gate electrodes of a capacitor505 and to a gate electrode of an EL driving TFT 506. Of two electrodesof the capacitor 505, one that is not connected to the switching TFT 504is connected to a power supply line V that is one of the power supplylines V1 to Vx. The EL driving TFT 506 has a source region and a drainregion one of which is connected to the power supply line V and theother of which is connected to an EL element 507.

In a pixel whose gate signal line G is selected, the signal electricpotential of the source signal line S is inputted to one of theelectrodes of the capacitor 505 through the switching TFT 504 that hasbeen turned conductive. The voltage between the electrodes of thecapacitor 505 is applied to the gate electrode of the EL driving TFT506. In accordance with this voltage applied, a current flows from thepower supply line V through the EL driving TFT 506 into the EL element507 and causes the EL element 507 to emit light.

The luminance of light emitted from the EL element 507 is almost inproportion with the amount of current flowing into the EL element 507.Therefore gray scales are obtained by changing the amount of currentflowing into the EL element 507.

In the display device shown in FIG. 5, the current flowing into the ELelement 507 is inputted from the power supply line V through the ELdriving TFT 506. The relation between a drain-source voltage VDS of aTFT and a drain current ID of the TFT in general is as shown in FIG. 8.

FIG. 8 is a graph showing plural ID curves obtained by varying the valueof a gate voltage ⁻V_(GS). The drain current I_(D) becomes larger as theabsolute value of the difference between the gate voltage V_(GS) and athreshold voltage V_(th) of the EL driving TFT 506 (|V_(GS)−V_(th)|)becomes larger, in other words, as the absolute value |V_(GS)| of thegate voltage V_(GS) becomes larger.

When the absolute value |V_(GS)−V_(th)| of the difference between thegate voltage V_(GS) and the threshold voltage V_(th) of the EL drivingTFT 506 is larger than the absolute value |V_(DS)| of the drain-sourcevoltage V_(DS), the TFT operates in a linear range. On the other hand,the TFT operates in a saturation range when |V_(GS)−V_(th)| is equal toor smaller than the absolute value |V_(DS)| of the drain-source voltageV_(DS).

The EL driving TFT 506 generally operates in the saturation range wherethe absolute value |V_(DS)| of the drain-source voltage V_(DS) is equalto or greater than the absolute value |V_(GS)−V_(th)| of the differencebetween the gate voltage V_(GS) and the threshold voltage V_(th) of theEL driving TFT 506.

In the saturation range, the drain current I_(D) of the TFT is inproportion to the second power of the gate voltage V_(GS) as shown inthe following Equation 1.

I _(D)=(1/2)μ₀ C ₀(W ² /L ²)(V _(GS) −V _(th))²  (Equation 1)

wherein, V_(th) represents the threshold voltage, μ₀ represents theeffective mobility, C₀ represents the capacitance of a gate insulatingfilm per unit area, W represents the gate width, and L represents thegate length.

In accordance with this equation, the electric potential to be inputtedto the source signal line S is changed such that the TFT receives a gatevoltage in proportion to the square root of the desired amount currentto be inputted to the EL element 507. In this way, the EL element iscaused to emit light of desired luminance.

When an image is to be displayed, an electric potential according to adesired gray scale is calculated by Equation 1 and is inputted to thesource signal line.

However, a video signal inputted from the external generally has ananalog electric potential that changes linearly with respect to theluminance obtained. Therefore accurate gray scales cannot be obtainedwhen a video signal supplied from the external is inputted to the signalline as it is.

There is a countermeasure in which an external correction circuitconverts the video signal into a drive signal in advance to suit thecharacteristics of the EL driving TFT and then the signal is sampled bythe source signal line driving circuit and outputted to pixels to obtaina given gray scale.

This measure, however, complicates the operation since it requires videosignal processing as above before the signal is inputted to the sourcesignal line driving circuit. Furthermore, the measure needs thecorrection circuit in addition to the source signal line driving circuitto obstruct reduction in size of the display device.

Accordingly, a method has to be found which makes it possible to obtaina given gray scale when a video signal is inputted directly to thesource signal line driving circuit.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above, and an objectof the present invention is therefore to provide a display device havinga source signal line driving circuit that makes a current flowingthrough an EL element of a pixel portion linearly correspond to a videosignal inputted from the external and then sampled.

A given gray scale thus can readily be obtained when a video signal isinputted directly.

The structure of the present invention will be shown below.

According to the present invention, there is provided a display devicehaving in each pixel an EL element, a source signal line, and a drivingTFT for driving the EL element, the device characterized by comprising:

means for converting an inputted analog signal voltage into a current;

means for converting the current into a gate-source voltage of a TFTthat has the same polarity as the driving TFT;

means for supplying the gate-source voltage as a source line signal tothe source signal line; and

means for supplying the source line signal to a gate electrode of thedriving TFT, and converting the source line signal into a current in thedriving TFT to drive the EL element.

According to the present invention, there is provided a display devicehaving in each pixel a source signal line, an EL driving TFT, a powersupply line, and an EL element, the source signal line inputting itssignal voltage to a gate electrode of the EL driving TFT, the powersupply line supplying a current to the EL element through thesource-drain of the EL driving TFT, the device characterized in that:

a correction TFT having the same polarity as the EL driving TFT isprovided;

an analog signal voltage sampled is converted into a signal currentlinearly corresponding thereto;

the signal current flows into the source-drain of the correction TFT tooutput, to the source signal line, a drive voltage obtained by addingthe gate-source voltage of the correction TFT to a reference electricpotential; and

the gate-source voltage of the EL driving TFT is substantially equal tothe gate-source voltage of the correction TFT when the electricpotential of the power supply line is set as the reference electricpotential.

According to the present invention, there is provided a display devicehaving in each pixel a source signal line, an EL driving TFT, a powersupply line, and an EL element, the source signal line inputting itssignal voltage to a gate electrode of the EL driving TFT, the powersupply line supplying a current to the EL element through thesource-drain of the EL driving TFT, the device characterized in that:

a signal input line, a switch, a resistor, a correction TFT, and anoperation amplifier are provided;

the correction TFT has the same polarity as the EL driving TFT;

the resistor has a first terminal and a second terminal;

the operation amplifier has a non-inversion input terminal, an inversioninput terminal, and an output terminal;

the signal input line is connected to the first terminal of the resistorthrough the switch;

the second terminal of the resistor is connected to the inversion inputterminal of the operation amplifier and to a source region or a drainregion of the correction TFT;

of the source region and the drain region of the correction TFT, the onethat is not connected to the inversion input terminal of the operationamplifier is connected to the output terminal of the operation amplifierand to the source signal line; and

a gate electrode of the correction TFT is connected to the drain regionor the source region of the correction TFT.

The display device may be characterized in that the electric potentialof the non-inversion input terminal of the operation amplifier is equalto the electric potential of the power supply line.

The display device may be characterized in that a switching TFT isprovided and that the source signal line is connected to a gateelectrode of the EL driving TFT through the source-drain of theswitching TFT.

The display device may be characterized in that a reset TFT is providedand that the reset TFT has a source region and a drain region one ofwhich is connected to the output terminal of the operation amplifier andthe other of which receives a given electric potential.

The present invention provides electronic equipment selected from apersonal computer, a video camera, a head mounted display, an imagereproducing device, and a portable information terminal, andcharacterized by comprising the display device described above.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram showing the structure of an EL displaydevice according to the present invention;

FIG. 2 is a graph showing the relation between an input voltage and theluminance of an EL element in the EL display device of the presentinvention;

FIG. 3 is a timing chart of the EL display device according to thepresent invention;

FIG. 4 is a diagram showing the structure of a conventional EL displaydevice;

FIG. 5 is a diagram showing the structure of a pixel of the conventionalEL display device;

FIG. 6 is a circuit diagram showing the structure of another EL displaydevice according to the present invention;

FIG. 7 is a timing chart of the EL display device according to thepresent invention;

FIG. 8 is a diagram showing the operation range of an EL driving TFT ofanalog gray scale system;

FIG. 9 is a circuit diagram showing the structure of still another ELdisplay device according to the present invention;

FIG. 10 is a timing chart of the EL display device according to thepresent invention;

FIG. 11 is a diagram showing the structure of yet still another ELdisplay device according to the present invention;

FIG. 12 is a timing chart of the EL display device according to thepresent invention;

FIG. 13A is a top view of the EL display device according to the presentinvention and FIGS. 13B and 13C are sectional views thereof;

FIGS. 14A to 14C are diagrams showing a process of manufacturing an ELdisplay device according to the present invention;

FIGS. 15A to 15C are diagrams showing the process of manufacturing an ELdisplay device according to the present invention;

FIGS. 16A and 16B are diagrams showing the process of manufacturing anEL display device according to the present invention;

FIG. 17 is a diagram showing the process of manufacturing an EL displaydevice according to the present invention;

FIGS. 18A to 18E are diagrams showing electronic equipment employing anEL display device of the present invention; and

FIG. 19 is a circuit diagram showing the structure of an EL displaydevice according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment Mode

The structure of the present invention will be described with referenceto FIG. 1.

FIG. 1 shows a pixel 115 of an EL display device according to thepresent invention, as well as a portion 118 of a source signal linedriving circuit for inputting a signal to the pixel.

The portion 118 of the source signal line driving circuit is composed ofa diode 101, a resistor 103, an operation amplifier 104, an analogsignal input line (signal input line) 107, a signal line 108, a switch(switching element) 109, and a reset TFT 117. The diode 101 consists ofa correction TFT 114 in which a gate electrode and a drain region areelectrically connected to each other.

The correction TFT 114 is a p-channel TFT. The reset TFT 117 may eitherbe a p-channel TFT or an n-channel TFT.

The pixel is composed of an EL driving TFT 102, a power supply line(power line) 105, a source signal line 106, a gate signal line 113, aswitching TFT 111, an EL element 112, and a capacitor 119. The ELdriving TFT 102 is a p-channel TFT. The switching TFT 111 may either bea p-channel TFT or an n-channel TFT.

The EL driving TFT and the correction TFT are both p-channel TFTs here,but n-channel TFTs may be used for the EL driving TFT and the correctionTFT. However, the EL driving TFT and the correction TFT have to have thesame polarity and almost the same threshold voltage.

The analog signal input line 107 is connected to the signal line 108,which is connected through the switch 109 to the resistor 103. Theresistor 103 is connected to an inversion input terminal 116 b of theoperation amplifier 104. The inversion input terminal 116 b of theoperation amplifier 104 is connected to a source region of thecorrection TFT 114. The operation amplifier 104 has a non-inversioninput terminal 116 a, to which a reference electric potential V_(ref) isinputted. An output terminal of the operation amplifier 104 is connectedto a drain region of the correction TFT 114 and to the source signalline 106. The reset TFT 117 has a source region and a drain region oneof which is connected to the source signal line 106 and the other ofwhich is grounded.

The switching TFT 111 has a gate electrode connected to the gate signalline 113. The switching TFT 111 also has a source region and a drainregion one of which is connected to the source signal line 106 and theother of which is connected to a gate electrode of the EL driving TFT102 and to one of two electrodes of the capacitor 119. The EL drivingTFT 102 has a source region connected to the power supply line 105 andhas a drain region connected to an anode of the EL element 112. Theother electrode of the capacitor 119 is connected to the power supplyline 105. A cathode of the EL element 112 is connected to a referencepower line, which is not shown in FIG. 1.

A method of driving the display device shown in FIG. 1 will bedescribed.

A signal voltage V_(in) of a video signal inputted to the analog signalinput line 107 is sampled when the switch 109 is opened or closed. Thesignal voltage is then inputted to the signal line 108.

The signal electric potential V_(in) here is equal to or larger than thereference electric potential V_(ref). A signal having a greater V_(in)value represents a higher luminance.

The inversion input terminal 116 b and the output terminal of theoperation amplifier 104 are connected to each other through the diode101. Therefore the electric potential of the inversion input terminal116 b is the same as the electric potential of the non-inversion inputterminal 116 a. In other words, the electric potential of the inversioninput terminal 116 b is equal to the reference electric potentialV_(ref). Then the voltage in the resistor 103 is V_(in)−V_(ref) and acurrent I₁ expressed as Equation 2 flows through the resistor 103.

I ₁=(V _(in) −V _(ref))/R  (Equation 2)

The current I₁ flows through the resistor from the side connected to theanalog signal input line 107 toward the side connected to the inversioninput terminal 116 b.

R represents the resistance of the resistor 103. The current I₁ isinputted to the diode 101. The drain current of the correction TFT 114that constitutes the diode 101 corresponds to the current I₁. Since thedrain region and the gate electrode are connected to each other in thecorrection TFT 114, the gate voltage of the TFT 114 is equal to thedrain-source voltage thereof. Therefore the correction TFT 114 operatesin the saturation range.

Being a p-channel TFT, the correction TFT 114 is not turned conductiveunless the electric potential of the correction TFT becomes higher onthe side connected to the inversion input terminal 116 b of theoperation amplifier 104 than the side connected to the output terminalof the operation amplifier 104.

Therefore the correction TFT on the side connected to the inversioninput terminal 116 b of the operation amplifier 104 serves as a sourceregion whereas the side connected to the output terminal of theoperation amplifier 104 serves as a drain region. The correction TFTthus functions as a diode that allows a current to flow in only onedirection.

Equation 1 given in the above is applicable to a TFT that operates inthe saturation range. The gate voltage is obtained by transformingEquation 1. Based on this transformation, the gate voltage V_(GS1) whenthe drain current is I₁ is obtained by Equation 3.

V _(GS1) =−{square root over (2I ₁(1/μ₀ C ₀)(L ₁ /W ₁))}+ V_(th1)  (Equation 3)

wherein, W₁ represents the gate width of the correction TFT 114, L₁represents the gate length of the correction TFT 114, and V_(th1)represents the threshold voltage of the correction TFT 114.

However, the gate voltage V_(GS1) and the threshold voltage V_(th1) areusually 0 or smaller since the correction TFT 114 here is a p-channelTFT.

Initially, the reset TFT 117 is conductive and the electric potential ofthe source signal line 106 is set to 0 V.

Then the reset TFT 117 is turned unconductive. Since the gate voltageand the drain-source voltage are equal to each other in the correctionTFT 114, an electric potential V_(ref)+V_(GS1) (the reference electricpotential shifted by the voltage V_(GS1)) is inputted to the sourcesignal line.

Before the electric potential V_(ref)+V_(GS1) is inputted to the sourcesignal line 106, the electric potential of the source signal line 106 isset to 0 V by turning the reset TFT 117 conductive. This is carried outin anticipation for the case in which the electric potential of thesource signal line 106 in a certain state is raised higher than theelectric potential of the inversion input terminal 116 b of theoperation amplifier 104 determined by a signal electric potential nextinputted to the analog signal input line 107. In this case, the sourceregion and the drain region interchanges their places in the correctionTFT 114 to make the correction TFT 114 unconductive and block feedbackbetween the input and output of the operation amplifier 104. Theinvention avoids this situation by setting the electric potential of thesource signal line to 0 V initially.

The electric potential given to the output terminal of the operationamplifier 104 when the reset TFT 117 is turned conductive is not limitedto 0 V. In general, the output terminal receives an electric potentialset equal to or lower than the lowest electric potential outputted tothe source signal line (hereinafter referred to as lowest electricpotential V_(SLOW)) which corresponds to the highest electric potentialof the signal inputted to the analog signal input line. In other words,the electric potential of the output terminal of the operation amplifier104 is set so as to be equal to or lower than the lowest electricpotential V_(SLOW) by turning the reset TFT 117 conductive.

The operation of setting the electric potential of the output terminalof the operation amplifier to the lowest electric potential V_(SLOW) sothat the correction TFT that is a p-channel TFT is always conductive iscalled a reset operation in this specification.

The reset operation can be carried out during the retrace period(horizontal retrace period) or other times.

The electric potential V_(ref)+V_(GS1) inputted to the source signalline 106 is inputted to the capacitor 119 and to the gate electrode ofthe EL driving TFT 102 through the switching TFT 111, which has beenturned conductive upon input of a signal to the gate signal line 113.The electric potential of the power supply line 105 (power supplyelectric potential) is set to the same level as the reference electricpotential V_(ref). Accordingly, the source region of the EL driving TFT102 in a conductive state has an electric potential equal to V_(ref).

At this point, a gate voltage V_(GS2) of the EL driving TFT 102 is equalto the gate voltage V_(GS1) of the correction TFT 114.

If the EL driving TFT 102 also operates in the saturation range,Equation 1 is applicable to the TFT 102. The drain current of the TFT102 in this case is given as I₂, which is obtained by Equation 4.

I ₂=(1/2)μ₀ C ₀(W ₂ /L ₂)(V _(GS1) −V _(th2))²  (Equation 4)

wherein, V_(th2) represents the threshold voltage of the EL driving TFT102, and W₂ and L₂ represent the gate width and the gate length of theEL driving TFT 102, respectively.

If the threshold voltage V_(th1) of the correction TFT 114 is almostequal to the threshold voltage V_(th2) of the EL driving TFT 102, thedrain current I₂ of the EL driving TFT 102 is independent of thethreshold voltage of the two TFTs as shown in Equation 5.

I ₂ =I ₁(W ₂ /L ₂)(L ₁ /W ₁)  (Equation 5)

In this way, the current I₂ that corresponds to the current I₁ linearlycan be inputted to the EL element 112.

Equation 2 shows that the current I₁ is in proportion to the inputelectric potential V_(in). FIG. 2 is a graph showing the relationbetween the signal electric potential V_(in) of a video signal and theluminance of the EL element in the EL display device structured asabove. The axis of abscissa indicates the signal electric potentialV_(in) of the video signal and the axis of ordinate indicates theluminance of the EL element. As the graph shows, the EL element 112 canemit light at a luminance linearly corresponding to the input voltageV_(in).

FIG. 3 is a timing chart illustrating the operation of the EL displaydevice structured as shown in FIG. 1. The EL display device has sourcesignal lines S1 to Sx (x lines in total), power supply lines V1 to Vx (xlines in total), and gate signal lines G1 to Gy (y lines in total).

The switching TFT and the reset TFT here are n-channel TFTs. Ifp-channel TFTs are used for the switching TFT and the reset TFT, thephase of the signals inputted to the gate signal lines G1 to Gy and tothe gate electrode of the reset TFT is reversed.

First, signals are inputted to the gate signal line G1 to turn everyswitching TFT that is connected to the gate signal line G1 conductive.The period during which the gate signal line G1 is selected is called afirst line period L1. In the first line period L1, signals inputted fromthe analog signal input line are sequentially inputted to the sourcesignal lines S1 to Sx. Each EL element emits light at a luminancecorresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, a signal Res is inputted to the gate electrode of the resetTFT to turn the reset TFT conductive, so that the electric potential isset to 0 V in all of the source signal lines S1 to Sx.

Thereafter, signals are inputted to the gate signal line G2 to turnevery switching TFT that is connected to the gate signal line G2conductive. Thus started is a second line period L2. Similar to thefirst line period L1, signals inputted from the analog signal input lineare sequentially inputted to the source signal lines S1 to Sx during thesecond line period L2. Each EL element emits light at a luminancecorresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, a signal Res is inputted to the gate electrode of the resetTFT to turn the reset TFT conductive, so that the electric potential isset to 0 V in all of the source signal lines S1 to Sx.

The same operation is repeated for all of the gate signal lines G1 to Gyto display one image. A period the display device takes to display oneimage is called one frame period. The operations above completes oneframe period F1.

After completion of the frame period F1, the gate signal line G1 isagain selected to start a second frame period F2.

The EL display device shown in FIG. 1 in accordance with the presentinvention displays images by repeating the above operations.

A buffer circuit 190 may be placed between the switch 109 and theresistor 103 as shown in FIG. 19. The buffer circuit 190 is composed ofa buffer 191 and a capacitor 192.

Embodiments of the present invention will be described below.

Embodiment 1

This embodiment describes with reference to FIG. 9 a case of usingn-channel TFTs for the correction TFT and the EL driving TFT in FIG. 1.Components in FIG. 9 that are identical with those in FIG. 1 are denotedby the same symbols.

FIG. 9 shows a pixel 115 of an EL display device according to thepresent invention, as well as a portion 118 of a source signal linedriving circuit for inputting a signal to the pixel.

The portion 118 of the source signal line driving circuit is composed ofa diode 101, a resistor 103, an operation amplifier 104, an analogsignal input line 107, a signal line 108, a switch 109, and a reset TFT117. The diode 101 consists of a correction TFT 914 in which a gateelectrode and a drain region are electrically connected to each other.

The correction TFT 914 is an n-channel TFT. The reset TFT 117 may eitherbe a p-channel TFT or an n-channel TFT.

The pixel 115 is composed of an EL driving TFT 902, a power supply line105, a source signal line 106, a switching TFT 111, an EL element 112, agate signal line 113, and a capacitor 119. The EL driving TFT 902 is ann-channel TFT. The switching TFT 111 may either be a p-channel TFT or ann-channel TFT.

The analog signal input line 107 is connected to the signal line 108,which is connected through the switch 109 to the resistor 103. Theresistor 103 is connected to an inversion input terminal 116 b of theoperation amplifier 104. The inversion input terminal 116 b of theoperation amplifier 104 is connected to a source region of thecorrection TFT 914. The operation amplifier 104 has a non-inversioninput terminal 116 a, to which a reference electric potential V_(ref) isinputted. An output terminal of the operation amplifier 104 is connectedto a drain region of the correction TFT 914 and to the source signalline 106. The reset TFT 117 has a source region and a drain region oneof which is connected to the source signal line 106 and the other ofwhich is grounded.

The switching TFT 111 has a gate electrode connected to the gate signalline 113. The switching TFT 111 also has a source region and a drainregion one of which is connected to the source signal line 106 and theother of which is connected to a gate electrode of the EL driving TFT902 and to one of two electrodes of the capacitor 119. The EL drivingTFT 902 has a source region connected to the power supply line 105 andhas a drain region connected to a cathode of the EL element 112. Theother electrode of the capacitor 119 is connected to the power supplyline 105. An anode of the EL element 112 is connected to a referencepower line, which is not shown in FIG. 9.

A method of driving the display device shown in FIG. 9 will bedescribed.

A signal voltage V_(in) of a video signal inputted to the analog signalinput line 107 is sampled when the switch 109 is opened or closed. Thesignal voltage is then inputted to the signal line 108.

In this embodiment, the signal electric potential V_(in) is equal to orsmaller than the reference electric potential V_(ref). A signal having asmaller V_(in) value represents a higher luminance.

The inversion input terminal 116 b and the output terminal of theoperation amplifier 104 are connected to each other through the diode101. Therefore the electric potential of the inversion input terminal116 b is the same as the electric potential of the non-inversion inputterminal 116 a. In other words, the electric potential of thenon-inversion input terminal 116 b is equal to the reference electricpotential V_(ref). Then, the voltage in the resistor 103 isV_(ref)−V_(in) and a current I₁ expressed as Equation 6 flows throughthe resistor 103.

I ₁=(V _(ref) −V _(in))/R  (Equation 6)

In this embodiment, the current I₁ flows through the resistor from theside connected to the inversion input terminal 116 b of the operationamplifier 104 toward the side connected to the analog signal input line107.

Herein, R represents the resistance of the resistor 103. The current I₁flows through the diode 101. The drain current of the correction TFT 914that constitutes the diode 101 corresponds to the current I₁. Since thedrain region and the gate electrode are connected to each other in thecorrection TFT 914, the gate voltage of the TFT 914 is equal to thedrain-source voltage thereof. Therefore the correction TFT 914 operatesin the saturation range.

Being an n-channel TFT, the correction TFT 914 is not turned conductiveunless the electric potential of the correction TFT becomes lower on theside connected to the inversion input terminal 116 b of the operationamplifier 104 than the side connected to the output terminal of theoperation amplifier 104.

Therefore the correction TFT 914 on the side connected to the inversioninput terminal 116 b of the operation amplifier 104 serves as a sourceregion whereas the side connected to the output terminal of theoperation amplifier 104 serves as a drain region. The correction TFTthus functions as a diode that allows a current to flow in only onedirection.

Equation 1 given in the above is applicable to a TFT that operates inthe saturation range. The gate voltage is obtained by transformingEquation 1. Based on this transformation, the gate voltage V_(GS1) whenthe drain current is I₁ is obtained by Equation 7.

V _(GS1) ={square root over (2I ₁(1/μ₀ C ₀)(L ₁ /W ₁))}+ V_(th1)  (Equation 7)

where, W₁ represents the gate width of the correction TFT 914, L₁represents the gate length of the correction TFT 914, and V_(th1)represents the threshold voltage of the correction TFT 914.

Initially, the reset TFT 117 is conductive and the electric potential ofthe source signal line 106 is set to 0 V.

Then the reset TFT 117 is turned unconductive. Since the gate voltageand the drain-source voltage are equal to each other in the correctionTFT 914, an electric potential V_(ref)+V_(GS1) (the reference electricpotential V_(ref) shifted by the voltage V_(GS1)) is inputted to thesource signal line 106.

Before the electric potential V_(ref)+V_(GS1) is inputted to the sourcesignal line 106, the electric potential of the source signal line 106 isset to 0 V by the reset TFT 117. This is carried out in anticipation forthe case in which the electric potential of the source signal line 106in a certain state is reduced lower than the electric potential of theinversion input terminal 116 b of the operation amplifier 104 changed bya signal electric potential inputted next. In this case, the sourceregion and the drain region interchange their places in the correctionTFT 914 to make the correction TFT 914 unconductive and block feedbackbetween the input and output of the operation amplifier 104. The presentinvention avoids this situation by setting the electric potential of thesource signal line to 0 V during the retrace period (horizontal retraceperiod).

The electric potential given to the output terminal of the operationamplifier 104 when the reset TFT 117 is turned conductive is not limitedto 0 V. In general, the output terminal receives an electric potentialset equal to or higher than the highest electric potential outputted tothe source signal line (hereinafter referred to as highest electricpotential V_(SHi)) which corresponds to the lowest electric potential ofthe signal inputted to the analog signal input line. In other words, theelectric potential of the output terminal of the operation amplifier 104is set so as to be equal to or higher than the highest electricpotential V_(SHi) by turning the reset TFT 117 conductive.

The operation of setting the electric potential of the output terminalof the operation amplifier to the highest electric potential so that thecorrection TFT that is an n-channel TFT is always conductive is called areset operation.

The electric potential V_(ref)+V_(GS1) inputted to the source signalline 106 is inputted to the capacitor 119 and to the gate electrode ofthe EL driving TFT 902 through the switching TFT 111, which has beenturned conductive upon input of a signal to the gate signal line 113.The electric potential of the power supply line 105 (power supplyelectric potential) is set to the same level as the reference electricpotential V_(ref). Accordingly, the source region of the EL driving TFT902 in a conductive state has an electric potential equal to V_(ref).

At this point, a gate voltage V_(GS2) of the EL driving TFT 902 is equalto the gate voltage V_(GS1) of the correction TFT 914. If the EL drivingTFT 902 also operates in the saturation range, Equation 1 is applicableto the TFT 902. The drain current of the TFT 902 in this case is givenas I₂, which is obtained by Equation 8.

I ₂=(1/2)/μ₀ C ₀(W ₂ /L ₂)(V _(GS1) −V _(th2))²  (Equation 8)

where, V_(th2) represents the threshold voltage of the EL driving TFT902, and W₂ and L₂ represent the gate width and the gate length of theEL driving TFT 902, respectively.

If the threshold voltage V_(th1) of the correction TFT 914 is almostequal to the threshold voltage V_(th2) of the EL driving TFT 902, thedrain current I₂ of the EL driving TFT 902 is independent of eachthreshold voltage of the two TFTs as shown in Equation 9.

I ₂ =I ₁(W ₂ /L ₂)(L ₁ /W ₁)  (Equation 9)

In this way, the current I₂ that corresponds to the current I₁ linearlycan be inputted to the EL element 112.

Equation 6 shows that the current I₁ is in proportion to the inputelectric potential V_(in). Thus the EL element 112 can emit light at aluminance linearly corresponding to the input potential V_(in).

FIG. 10 is a timing chart illustrating the operation of the EL displaydevice structured as shown in FIG. 9. The EL display device has sourcesignal lines S1 to Sx (x lines in total), power supply lines V1 to Vx (xlines in total), and gate signal lines G1 to Gy (y lines in total).

The switching TFT 111 and the reset TFT 117 here are n-channel TFTs. Ifp-channel TFTs are used for the switching TFT and the reset TFT, thephase of the signals inputted to the gate signal lines G1 to Gy and tothe gate electrode of the reset TFT 117 is reversed.

First, signals are inputted to the gate signal line G1 to turn everyswitching TFT 111 that is connected to the gate signal line G1conductive. The period during which the gate signal line G1 is selectedis called a first line period L1. In the first line period L1, signalsinputted from the analog signal input line 107 are sequentially inputtedto the source signal lines S1 to Sx. Each EL element 112 emits light ata luminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, a signal Res is inputted to the gate electrode of the resetTFT 117 to turn the reset TFT 117 conductive, so that the electricpotential is set to 0 V in all of the source signal lines S1 to Sx.

Thereafter, signals are inputted to the gate signal line G2 to turnevery switching TFT 111 that is connected to the gate signal line G2conductive. Thus started is a second line period L2. Similar to thefirst line period L1, signals inputted from the analog signal input line107 are sequentially inputted to the source signal lines S1 to Sx duringthe second line period L2. Each EL element 112 emits light at aluminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, a signal Res is inputted to the gate electrode of the resetTFT to turn the reset TFT conductive, so that the electric potential isset to 0 V in all of the source signal lines S1 to Sx.

The same operation is repeated for all of the gate signal lines G1 to Gyto display one image. A period the display device takes to display oneimage is called one frame period. The operation above completes oneframe period F1.

After completion of the frame period F1, the gate signal line G1 isagain selected to start a second frame period F2.

The EL display device shown in FIG. 9 in accordance with the presentinvention displays images by repeating the above operations.

Embodiment 2

This embodiment describes a driving circuit having a structure differentfrom the one in FIG. 1.

FIG. 6 shows the structure of the driving circuit according to thisembodiment. Components in FIG. 6 that are identical with those in FIG. 1are denoted by the same symbols and explanations thereof are omitted.Unlike FIG. 1, the reset TFT 117 is not provided in FIG. 6.

For that reason, measures other than the reset TFT are taken to reducethe electric potential of the source signal line 106 prior to input of asignal to a level lower than the electric potential of the inversioninput terminal 116 b of the operation amplifier 104 determined by avideo signal inputted next. The period assigned to reduce the electricpotential of the source signal line 106 prior to input of a signal to alevel lower than the electric potential of the inversion input terminal116 b of the operation amplifier 104 determined by a video signalinputted next, is called a reset period.

In FIG. 6, an electric potential V₊ inputted to the non-inversion inputterminal 106 a of the operation amplifier 104 is kept at the referenceelectric potential V_(ref) while a video signal is sampled. On the otherhand, V₊ is reduced to an electric potential V_(LOW) during the resetperiod. The electric potential V_(LOW) is always set to a level lowerthan the electric potential of the inversion input terminal 116 b of theoperation amplifier 104 whatever electric potential the inversion inputterminal takes due to a video signal inputted from the external.

Therefore the electric potential V_(LOW) is set to a level equal to orlower than the reference electric potential V_(ref).

In the reset period, the electric potential of the non-inversion inputterminal 106 a of the operation amplifier 104 is equal to the electricpotential V_(LOW).

Then the electric potential of the non-inversion input terminal 116 a islower than the electric potential of the inversion input terminal 1016 bto cause the operation amplifier 104 to output a low power supplyelectric potential. In this way, the output terminal of the operationamplifier is kept to a sufficiently low electric potential.

The electric potential of the source signal line 106 is thus reduced toa level equal to or lower than V_(LOW).

After the reset period, the electric potential of the non-inversioninput terminal 116 a of the operation amplifier 104 is returned toV_(ref) and then a video signal is inputted. Since the electricpotential of the inversion input terminal 116 b of the operationamplifier 104 is always higher than the electric potential V_(LOW), thediode 101 keeps operating in a conductive state (without beingaccidentally turned unconductive) to effect feedback between the inputand output of the operation amplifier 104. The operation amplifier thusoperates so as to equalize the electric potential of the non-inversioninput terminal 116 a with the electric potential of the inversion inputterminal 116 b.

The reset period is provided within the retrace period (horizontalretrace period).

The other operations are the same as those in Embodiment Mode.

FIG. 7 is a timing chart illustrating the operation of the EL displaydevice structured as shown in FIG. 6. The EL display device has sourcesignal lines S1 to Sx (x lines in total), power supply lines V1 to Vx (xlines in total), and gate signal lines G1 to Gy (y lines in total).

The switching TFT 111 here is an n-channel TFT. If a p-channel TFT isused for the switching TFT, the phase of the signal inputted to the gatesignal lines G1 to Gy is reversed.

First, signals are inputted to the gate signal line G1 to turn everyswitching TFT 111 that is connected to the gate signal line G1conductive. The period during which the gate signal line G1 is selectedis called a first line period L1. In the first line period L1, signalsinputted from the analog signal input line 107 are sequentially inputtedto the source signal lines S1 to Sx. Each EL element 112 emits light ata luminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period (horizontal retrace period) Lb isprovided in order to input signals to the source signal lines againstarting from S1. During the retrace period Lb, the electric potentialV_(LOW) is inputted to the non-inversion input terminal 116 a of theoperation amplifier 104 to set the electric potential to a level equalto or lower than V_(LOW) in all of the source signal lines S1 to Sx.

Thereafter, signals are inputted to the gate signal line G2 to turnevery switching TFT 111 that is connected to the gate signal line G2conductive. Thus started is a second line period L2. Similar to thefirst line period L1, signals inputted from the analog signal input line107 are sequentially inputted to the source signal lines S1 to Sx duringthe second line period L2. Each EL element 112 emits light at aluminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, the electric potential V_(LOW) is inputted to thenon-inversion input terminal 116 a of the operation amplifier 104 to setthe electric potential to a level equal to or lower than V_(LOW) in allof the source signal lines S1 to Sx.

The same operation is repeated for all of the gate signal lines G1 to Gyto display one image. A period the display device takes to display oneimage is called one frame period. The operations above complete a firstframe period F1.

After completion of the first frame period F1, the gate signal line G1is again selected to start a second frame period F2.

The EL display device shown in FIG. 6 in accordance with the presentinvention displays images by repeating the above operations.

Embodiment 3

This embodiment describes a driving circuit having a structure differentfrom the one in FIG. 9.

FIG. 11 shows the structure of the driving circuit according to thisembodiment. Components in FIG. 11 that are identical with those in FIG.9 are denoted by the same symbols and explanations thereof are omitted.Unlike in FIG. 9, no reset TFT 117 is provided in FIG. 11.

For that reason, measures other than the reset TFT are taken to raisethe electric potential of the source signal line 106 prior to input of asignal to a level higher than the electric potential of the inversioninput terminal 116 b of the operation amplifier 104 determined by avideo signal inputted next. The period assigned to raise the electricpotential of the source signal line 106 prior to input of a signal to alevel higher than the electric potential of the inversion input terminal116 b of the operation amplifier 104 determined by a video signalinputted next, is called a reset period.

In FIG. 11, an electric potential V₊ inputted to the non-inversion inputterminal 106 a of the operation amplifier 104 is kept at the referenceelectric potential V_(ref) while a video signal is sampled. On the otherhand, V₊ is raised to an electric potential V_(Hi) during the resetperiod. The electric potential V_(Hi) is always set to a level higherthan the electric potential of the inversion input terminal 116 b of theoperation amplifier 104 whatever electric potential the inversion inputterminal takes due to a video signal inputted from the external.

In other words, the electric potential V_(Hi) is set to a level equal toor higher than the reference electric potential V_(ref).

Then the electric potential of the non-inversion input terminal 106 a ishigher than the electric potential of the inversion input terminal 106 bto cause the operation amplifier 104 to output a high power supplyelectric potential. In this way, the output terminal of the operationamplifier is kept to a sufficiently high electric potential.

In the reset period, the electric potential of the output terminal ofthe operation amplifier 104 is equal to or higher than V_(Hi). After thereset period, the electric potential of the non-inversion terminal 116 aof the operation amplifier 104 is returned to V_(ref) and then a videosignal is inputted. Since the electric potential of the inversion inputterminal 116 b of the operation amplifier 104 is always lower than theelectric potential V_(Hi), the diode 101 keeps operating in a conductivestate (without being accidentally turned unconductive) to effectfeedback between the input and output of the operation amplifier 104.The operation amplifier thus operates so as to equalize the electricpotential of the non-inversion input terminal 116 a with the electricpotential of the inversion input terminal 116 b.

The reset period is provided within the retrace period (horizontalretrace period).

The other operations are the same as those in Embodiment 1.

FIG. 12 is a timing chart illustrating the operation of the EL displaydevice structured as shown in FIG. 11. The EL display device has sourcesignal lines S1 to Sx (x lines in total), power supply lines V1 to Vx (xlines in total), and gate signal lines G1 to Gy (y lines in total).

The switching TFT 111 here is an n-channel TFT. If a p-channel TFT isused for the switching TFT, the phase of the signal inputted to the gatesignal lines G1 to Gy is reversed.

First, signals are inputted to the gate signal line G1 to turn everyswitching TFT 111 that is connected to the gate signal line G1conductive. The period during which the gate signal line G1 is selectedis called a first line period L1. In the first line period L1, signalsinputted from the analog signal input line 107 are sequentially inputtedto the source signal lines S1 to Sx. Each EL element 112 emits light ata luminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period (horizontal retrace period) Lb isprovided in order to input signals to the source signal lines againstarting from S1. During the retrace period Lb, the electric potentialV_(Hi) is inputted to the non-inversion input terminal 116 a of theoperation amplifier 104 to set the electric potential to a level equalto or higher than V_(Hi) in all of the source signal lines S1 to Sx.

Thereafter, signals are inputted to the gate signal line G2 to turnevery switching TFT 111 that is connected to the gate signal line G2conductive. Thus started is a second line period L2. Similar to thefirst line period L1, signals inputted from the analog signal input line107 are sequentially inputted to the source signal lines S1 to Sx duringthe second line period L2. Each EL element 112 emits light at aluminance corresponding to the inputted signal electric potential.

After inputting the signals to all of the source signal lines S1 to Sxis completed, a retrace period Lb is provided in order to input signalsto the source signal lines again starting from S1. During the retraceperiod Lb, the electric potential V_(Hi) is inputted to thenon-inversion input terminal 116 a of the operation amplifier 104 to setthe electric potential to a level equal to or higher than V_(Hi) in allof the source signal lines S1 to Sx.

The same operation is repeated for all of the gate signal lines G1 to Gyto display one image. A period the display device takes to display oneimage is called one frame period. The operations above complete a firstframe period F1.

After completion of the first frame period F1, the gate signal line G1is again selected to start a second frame period F2.

The EL display device shown in FIG. 11 in accordance with the presentinvention displays images by repeating the above operations.

Embodiment 4

In this embodiment, a method of simultaneously manufacturing TFTs(n-channel TFT and p-channel TFT) in a pixel portion and a drivercircuit provided in the periphery of the pixel portion on the samesubstrate in the EL display of the present invention is described indetail with reference to FIGS. 14 to 17.

First, in this embodiment, a substrate 300 is used, which is made ofglass such as barium borosilicate glass or alumino borosilicate glass,typified by #7059 glass or #1737 glass of Corning Inc. There is nolimitation on the substrate 300 as long as a substrate having a lighttransmitting property is used, and a quartz substrate may also be used.In addition, a plastic substrate having heat resistance to a treatmenttemperature of this embodiment may also be used.

Then, a base film 301 formed of an insulating film such as a siliconoxide film, a silicon nitride film or a silicon oxynitride film isformed on the substrate 300. In this embodiment, a two-layer structureis used for the base film 301. However, a single layer film or alamination structure consisting of two or more layers of the insulatingfilm may also be used. As a first layer of the base film 301, a siliconoxynitride film 301 a is formed with a thickness of 10 to 200 nm(preferably 50 to 100 nm) using SiH₄, NH₃, and N₂O as reaction gases bya plasma CVD method. In this embodiment, the silicon oxynitride film 301a (composition ratio Si=32%, O=27%, N=24% and H=17%) having a filmthickness of 50 nm is formed. Then, as a second layer of the base film301, a silicon oxynitride film 301 b is formed so as to be laminated onthe first layer with a thickness of 50 to 200 nm (preferably 100 to 150nm) using SiH₄ and N₂O as reaction gases by the plasma CVD method. Inthis embodiment, the silicon oxynitride film 301 b (composition ratioSi=32%, O=59%, N=7% and H=2%) having a film thickness of 100 nm isformed.

Subsequently, semiconductor layers 302 to 305 are formed on the basefilm. The semiconductor layers 302 to 305 are formed such that asemiconductor film having an amorphous structure is formed by a knownmethod (a sputtering method, an LPCVD method, a plasma CVD method or thelike), and is subjected to a known crystallization process (a lasercrystallization method, a thermal crystallization method, a thermalcrystallization method using a catalyst such as nickel, or the like) toobtain a crystalline semiconductor film, and the crystallinesemiconductor film is patterned into desired shapes. The semiconductorlayers 302 to 305 are formed with a thickness of 25 to 80 nm (preferably30 to 60 nm). The material of the crystalline semiconductor film is notparticularly limited, but it is preferable to form the film usingsilicon, a silicon germanium (Si_(x)Ge_(1-x)(X=0.0001 to 0.02)) alloy,or the like. In this embodiment, an amorphous silicon film of 55 nmthickness is formed by a plasma CVD method, and then, anickel-containing solution is held on the amorphous silicon film. Adehydrogenation process of the amorphous silicon film is performed (at500° C. for 1 hour), and thereafter a thermal crystallization process isperformed (at 550° C. for 4 hours) thereto. Further, to improve thecrystallinity, a laser annealing process is performed to form thecrystalline silicon film. Then, this crystalline silicon film issubjected to a patterning process using a photolithography method toobtain the semiconductor layers 302 to 305.

Further, after the formation of the semiconductor layers 302 to 305, aminute amount of impurity element (boron or phosphorus) may be doped tocontrol a threshold value of the TFT.

Besides, in the case where the crystalline semiconductor film ismanufactured by the laser crystallization method, a pulse oscillationtype or continuous emission type excimer laser, YAG laser, or YVO₄ lasermay be used. In the case where those lasers are used, it is appropriateto use a method in which laser light radiated from a laser oscillator iscondensed into a linear shape by an optical system, and is irradiated tothe semiconductor film. Although the conditions of crystallizationshould be properly selected by an operator, in the case where theexcimer laser is used, a pulse oscillation frequency is set to 30 Hz,and a laser energy density is set to 100 to 400 mJ/cm² (typically 200 to300 mJ/cm²). In the case where the YAG laser is used, it is appropriateto set a pulse oscillation frequency as 1 to 10 Hz using the secondharmonic, and to set a laser energy density to 300 to 600 mJ/cm²(typically, 350 to 500 mJ/cm²). Then, laser light condensed into alinear shape with a width of 100 to 1000 μm, for example, 400 μm, isirradiated to the whole surface of the substrate, and an overlappingratio (overlap ratio) of the linear laser light at this time may be setto 50 to 90%.

A gate insulating film 306 is then formed for covering the semiconductorlayers 302 to 305. The gate insulating film 306 is formed of aninsulating film containing silicon with a thickness of 40 to 150 nm by aplasma CVD or sputtering method. In this embodiment, the gate insulatingfilm 306 is formed of a silicon oxynitride film with a thickness of 110nm by the plasma CVD method (composition ratio Si=32%, O=59%, N=7%, andH=2%). Of course, the gate insulating film is not limited to the siliconoxynitride film, and other insulating films containing silicon may beused with a single layer or a lamination structure.

Besides, when a silicon oxide film is used, it can be formed such thatTEOS (tetraethyl orthosilicate) and O₂ are mixed by the plasma CVDmethod with a reaction pressure of 40 Pa and a substrate temperature of300 to 400° C., and discharged at a high frequency (13.56 MHz) powerdensity of 0.5 to 0.8 W/cm². The silicon oxide film thus manufacturedcan obtain satisfactory characteristics as the gate insulating film bysubsequent thermal annealing at 400 to 500° C.

Then, as shown in FIG. 14A, a first conductive film 307 of 20 to 100 nmthickness and a second conductive film 308 of 100 to 400 nm thicknessare formed into lamination on the gate insulating film 306. In thisembodiment, the first conductive film 307 made of a TaN film with athickness of 30 nm and the second conductive film 308 made of a W filmwith a thickness of 370 nm are formed into lamination. The TaN film isformed by sputtering with a Ta target under a nitrogen containingatmosphere. Besides, the W film is formed by sputtering with a W target.The W film may also be formed by a thermal CVD method using tungstenhexafluoride (WF₆). Whichever method is used, it is necessary to makethe material have low resistance for use as a gate electrode, and it ispreferred that the resistivity of the W film is set to 20 μΩcm or less.It is possible to make the W film have low resistance by making thecrystal grains large. However, in the case where many impurity elementssuch as oxygen are contained within the W film, crystallization isinhibited and the resistance becomes higher. Therefore, in thisembodiment, the W film is formed by sputtering using a W target having ahigh purity of 99.9999%, and also by taking sufficient consideration soas to prevent impurities within the gas phase from mixing therein duringthe film formation, and thus, a resistivity of 9 to 20 μΩcm can berealized.

Note that, in this embodiment, the first conductive film 307 is made ofTaN, and the second conductive film 308 is made of W, but the materialis not particularly limited thereto, and either film may be formed froman element selected from the group consisting of Ta, W, Ti, Mo, Al, Cu,Cr, and Nd or an alloy material or a compound material containing theabove element as its main constituent. Besides, a semiconductor filmtypified by a polycrystalline silicon film doped with an impurityelement such as phosphorus may be used. An alloy made of Ag, Pd, and Cumay also be used. Further, any combination may be employed such as acombination in which the first conductive film is formed of a tantalum(Ta) film and the second conductive film is formed of a W film, acombination in which the first conductive film is formed of a titaniumnitride (TiN) film and the second conductive film is formed of a W film,a combination in which the first conductive film is formed of a tantalumnitride (TaN) film and the second conductive film is formed of an Alfilm, or a combination in which the first conductive film is formed of atantalum nitride (TaN) film and the second conductive film is formed ofa Cu film.

Next, as shown in FIG. 14B, masks 309 to 313 made of resist are formedby using a photolithography method, and a first etching process forforming electrodes and wirings is carried out. In the first etchingprocess, first and second etching conditions are used. In thisembodiment, as the first etching condition, an ICP (inductively coupledplasma) etching method is used, in which CF₄, Cl₂, and O₂ are used asetching gases, a gas flow rate is set to 25/25/10 sccm, and an RF (13.56MHz) power of 500 W is applied to a coil shape electrode under apressure of 1 Pa to generate plasma. Thus, the etching is performed. Adry etching device using ICP (Model E645-ICP) manufactured by MatsushitaElectric Industrial Co. is used here. A 150 W RF (13.56 MHz) power isalso applied to the substrate side (sample stage), thereby substantiallyapplying a negative self-bias voltage. The W film is etched under thefirst etching condition, and the end portion of the first conductivelayer is formed into a tapered shape. In the first etching condition,the etching rate for W is 200.39 nm/min, the etching rate for TaN is80.32 nm/min, and the selectivity of W to TaN is about 2.5. Further, thetaper angle of W is about 26° under the first etching condition.

Thereafter, as shown in FIG. 14B, the etching condition is changed intothe second etching condition without removing the masks 309 to 313 madeof resist, and the etching is performed for about 30 seconds, in whichCF₄ and Cl₂ are used as the etching gases, a gas flow rate is set to30/30 sccm, and an RF (13.56 MHz) power of 500 W is applied to a coilshape electrode under a pressure of 1 Pa to generate plasma. An RF(13.56 MHz) power of 20 W is also applied to the substrate side (samplestage), and a substantially negative self-bias voltage is appliedthereto. In the second etching condition in which CF₄ and Cl₂ are mixed,the W film and the TaN film are etched to the same degree. In the secondetching condition, the etching rate for W is 58.97 nm/min, and theetching rate for TaN is 66.43 nm/min. Note that, in order to perform theetching without leaving any residue on the gate insulating film, it isappropriate that an etching time is increased by approximately 10 to20%.

In the above first etching process, by making the shapes of the masksformed of resist suitable, end portions of the first conductive layerand the second conductive layer become tapered shape by the effect ofthe bias voltage applied to the substrate side. The angle of the taperportion may be 15 to 45°. In this way, first shape conductive layers 314to 318 consisting of the first conductive layer and the secondconductive layer (first conductive layers 314 a to 318 a and secondconductive layers 314 b to 318 b) are formed by the first etchingprocess. Reference numeral 319 indicates a gate insulating film, and theregions not covered with the first shape conductive layers 314 to 318are made thinner by approximately 20 to 50 nm by etching.

Then, a first doping process is performed to add an impurity elementimparting n-type conductivity to the semiconductor layer withoutremoving the masks made of resist (FIG. 14B). Doping may be carried outby an ion doping method or an ion injecting method. The condition of theion doping method is that a dosage is 1×10¹³ to 5×10¹⁵ atoms/cm², and anacceleration voltage is 60 to 100 keV. In this embodiment, the dosage is1.5×10¹⁵ atoms/cm² and the acceleration voltage is 80 keV. As theimpurity element imparting n-type conductivity, an element belonging togroup 15 of the periodic table, typically phosphorus (P) or arsenic (As)is used, but phosphorus (P) is used here. In this case, the conductivelayers 314 to 318 become masks for the impurity element imparting n-typeconductivity, and high concentration impurity regions 320 to 323 areformed in a self-aligning manner. The impurity element imparting n-typeconductivity in a concentration range of 1×10²⁰ to 1×10²¹ atoms/cm³ isadded to the high concentration impurity regions 320 to 323.

Thereafter, as shown in FIG. 14C, a second etching process is performedwithout removing the masks made of resist. Here, a gas mixture of CF₄,Cl₂ and O₂ is used as an etching gas, the gas flow rate is set to20/20/20 sccm, and a 500 W RF (13.56 MHz) power is applied to a coilshape electrode under a pressure of 1 Pa to generate plasma, therebyperforming etching. A 20 W RF (13.56 MHz) power is also applied to thesubstrate side (sample stage), thereby substantially applying a negativeself-bias voltage. In the second etching process, the etching rate for Wis 124.62 nm/min, the etching rate for TaN is 20.67 nm/min, and theselectivity of W to TaN is 6.05. Accordingly, the W film is selectivelyetched. The taper angle of W is 70° by the second etching process.Second conductive layers 324 b to 328 b are formed by the second etchingprocess. On the other hand, the first conductive layers 314 a to 318 aare hardly etched, and first conductive layers 324 a to 328 a areformed.

Next, as shown in FIG. 15A, a second doping process is performed. Thesecond conductive layers 324 b to 328 b are used as masks for animpurity element, and doping is performed such that the impurity elementis added to the semiconductor layer below the tapered portions of thefirst conductive layers. In this embodiment, phosphorus (P) is used asthe impurity element, and plasma doping is performed with a dosage of1.5×10¹⁴ atoms/cm², a current density of 0.5 μA, and an accelerationvoltage of 90 keV. Thus, low concentration impurity regions 329 to 333,which overlap with the first conductive layers, are formed inself-aligning manner The concentration of phosphorus (P) added to thelow concentration impurity regions 329 to 333 is 1×10¹⁷ to 5×10¹⁸atoms/cm³, and has a gentle concentration gradient in accordance withthe film thickness of the tapered portions of the first conductivelayers. Note that in the semiconductor layers that overlap with thetapered portions of the first conductive layers, the concentration ofthe impurity element slightly falls from the end portions of the taperedportions of the first conductive layers toward the inner portions, butthe concentration keeps almost the same level. Further, an impurityelement is added to the high concentration impurity regions 333 to 337to form high concentration impurity regions 333 to 337.

Thereafter, as shown in FIG. 15B, after the masks made of resist areremoved, a third etching process is performed using a photolithographymethod. The tapered portions of the first conductive layers arepartially etched so as to have shapes overlapping the second conductivelayers in the third etching process. Incidentally, as shown in FIG. 15B,masks made of resist (338 and 339) are formed in the regions where thethird etching process is not conducted.

The etching condition in the third etching process is that Cl₂ and SF₆are used as etching gases, the gas flow rate is set to 10/50 sccm, andthe ICP etching method is used as in the first and second etchingprocesses. Note that, in the third etching process, the etching rate forTaN is 111.2 nm/min, and the etching rate for the gate insulating filmis 12.8 nm/min.

In this embodiment, a 500 W RF (13.56 MHz) power is applied to a coilshape electrode under a pressure of 1.3 Pa to generate plasma, therebyperforming etching. A 10 W RF (13.56 MHz) power is also applied to thesubstrate side (sample stage), thereby substantially applying a negativeself-bias voltage. Thus, first conductive layers 340 a to 342 a areformed.

Impurity regions (LDD regions) 343 to 345, which do not overlap thefirst conductive layers 340 a to 342 a, are formed by the third etchingprocess. Note that impurity regions (GOLD regions) 346 and 347 remainsoverlapping the first conductive layers 324 a and 326 a.

Further, the electrode constituted of the first conductive layer 324 aand the second conductive layer 324 b finally becomes the gate electrodeof the n-channel TFT of the driver circuit, and the electrodeconstituted of the first conductive layer 340 a and a second conductivelayer 340 b finally becomes the gate electrode of the p-channel TFT ofthe driver circuit.

Similarly, the electrode constituted of the first conductive layer 341 aand a second conductive layer 341 b finally becomes the gate electrodeof the n-channel TFT of the pixel portion, and the electrode constitutedof the first conductive layer 342 a and a second conductive layer 342 bfinally becomes the gate electrode of the p-channel TFT of the pixelportion. Further, the electrode constituted of the first conductivelayer 326 a and the second conductive layer 326 b finally becomes one ofelectrodes of a capacitor (storage capacitor) of the pixel portion.

In this way, in this embodiment, the impurity regions (LDD regions) 343to 345 that do not overlap the first conductive layers 340 a to 342 aand the impurity regions (GOLD regions) 346 and 347 that overlap thefirst conductive layers 324 a and 326 a can be simultaneously formed.Thus, different impurity regions can be formed in accordance with theTFT characteristics.

Next, the gate insulating film 319 is subjected to an etching process.In this etching process, CHF₃ is used as an etching gas, and a reactiveion etching method (RIE method) is used. In this embodiment, a thirdetching process is conducted with a chamber pressure of 6.7 Pa, RF powerof 800 W, and a gas flow rate of CHF₃ of 35 sccm.

Thus, parts of the high concentration impurity regions 333 to 337 areexposed, and insulating films 356 a to 356 e are formed.

Subsequently, after the masks made of resist are removed, masks 348 and349 made of resist are newly formed to thereby perform a third dopingprocess. By this third doping process, impurity regions 350 to 355 addedwith an impurity element imparting conductivity (p-type) opposite to theabove conductivity (n-type) are formed in the semiconductor layers thatbecome active layers of the p-channel TFT (FIG. 15C). The firstconductive layers 340 a, 326 a, and 342 a are used as masks for theimpurity element, and the impurity element imparting p-type conductivityis added to form the impurity regions in a self-aligning manner.

In this embodiment, the impurity regions 350 to 355 are formed by an iondoping method using diborane (B₂H₆). Note that, in the third dopingprocess, the semiconductor layers forming the n-channel TFTs are coveredwith the masks 348 and 349 made of resist. The impurity regions 350 to355 are respectively added with phosphorous at different concentrationsby the first doping process and the second doping process. In any of theregions, the doping process is conducted such that the concentration ofthe impurity element imparting p-type conductivity becomes 2×10²⁰ to2×10²¹ atoms/cm³. Thus, the impurity regions function as source anddrain regions of the p-channel TFT, and therefore, no problem occurs.

Through the above-described processes, the impurity regions are formedin the respective semiconductor layers.

Note that, in this embodiment, a method of conducting doping of theimpurities (boron) after etching the gate insulating film is shown, butdoping of the impurities may be conducted before etching the gateinsulating film.

Subsequently, the masks 348 and 349 made of resist are removed, and asshown in FIG. 16A, a first interlayer insulating film 357 is formed. Asthe first interlayer insulating film 357, an insulating film containingsilicon is formed with a thickness of 100 to 200 nm by a plasma CVDmethod or a sputtering method. In this embodiment, a silicon oxynitridefilm of 150 nm thickness is formed by the plasma CVD method. Of course,the first interlayer insulating film 357 is not limited to the siliconoxynitride film, and other insulating films containing silicon may beused in a single layer or a lamination structure.

Then, a process of activating the impurity element added to thesemiconductor layers is performed. This activation process is performedby a thermal annealing method using an annealing furnace. The thermalannealing method may be performed in a nitrogen atmosphere with anoxygen concentration of 1 ppm or less, preferably 0.1 ppm or less and at400 to 700° C., typically 500 to 550° C. In this embodiment, theactivation process is conducted by a heat treatment for 4 hours at 550°C. Note that, in addition to the thermal annealing method, a laserannealing method or a rapid thermal annealing method (RTA method) can beapplied.

Note that, in this embodiment, with the activation process, nickel usedas a catalyst in crystallization is gettered to the impurity regionscontaining phosphorous at high concentration, and the nickelconcentration in the semiconductor layer that becomes a channel formingregion is mainly reduced. The TFT thus manufactured having the channelforming region has the lowered off current value and good crystallinityto obtain a high electric field effect mobility. Thus, the satisfactorycharacteristics can be attained.

Further, the activation process may be conducted before the formation ofthe first interlayer insulating film. Incidentally, in the case wherethe used wiring material is weak to heat, the activation process ispreferably conducted after the formation of the interlayer insulatingfilm (insulating film containing silicon as its main constituent, forexample, silicon nitride film) in order to protect wirings and the likeas in this embodiment.

Furthermore, after the activation process and the doping process, thefirst interlayer insulating film may be formed.

Moreover, a heat treatment is carried out at 300 to 550° C. for 1 to 12hours in an atmosphere containing hydrogen of 3 to 100% to perform aprocess of hydrogenating the semiconductor layers. In this embodiment,the heat treatment is conducted at 410° C. for 1 hour in a nitrogenatmosphere containing hydrogen of approximately 3%. This is a process ofterminating dangling bonds in the semiconductor layer by hydrogenincluded in the interlayer insulating film. As another means forhydrogenation, plasma hydrogenation (using hydrogen excited by plasma)may be performed.

In addition, in the case where the laser annealing method is used as theactivation process, after the hydrogenation process, laser light emittedfrom an excimer laser, a YAG laser or the like is desirably irradiated.

Next, as shown in FIG. 16B, a second interlayer insulating film 358,which is made from an organic insulating material, is formed on thefirst interlayer insulating film 357. In this embodiment, an acrylicresin film is formed with a thickness of 1.6 μm. Then, patterning forforming contact holes that reach the respective impurity regions 333,336, 350 and 352 is conducted.

As the second interlayer insulating film 358, a film made from aninsulating material containing silicon or an organic resin is used. Asthe insulating material containing silicon, silicon oxide, siliconnitride, or silicon oxynitride may be used. As the organic resin,polyimide, polyamide, acrylic, BCB (benzocyclobutene), or the like maybe used.

In this embodiment, the silicon oxynitride film formed by a plasma CVDmethod is formed. Note that the thickness of the silicon oxynitride filmis preferably 1 to 5 μm (more preferably 2 to 4 μm). The siliconoxynitride film has a little amount of moisture contained in the filmitself, and thus, is effective in suppressing deterioration of the ELelement.

Further, dry etching or wet etching may be used for the formation of thecontact holes. However, taking the problem of electrostatic destructionin etching into consideration, the wet etching method is desirably used.

Moreover, in the formation of the contact holes here, the firstinterlayer insulating film and the second interlayer insulating film areetched at the same time. Thus, in consideration for the shape of thecontact hole, it is preferable that the material with an etching speedfaster than that of the material for forming the first interlayerinsulating film is used for the material for forming the secondinterlayer insulating film.

Then, wirings 359 to 366, which are electrically connected with theimpurity regions 333, 336, 350, and 352, respectively, are formed. Thewirings are formed by patterning a lamination film of a Ti film of 50 nmthickness and an alloy film (alloy film of Al and Ti) of 500 nmthickness, but other conductive films may also be used.

Subsequently, a transparent conductive film is formed thereon with athickness of 80 to 120 nm, and by patterning the transparent conductivefilm, a transparent electrode 367 is formed (FIG. 16B).

Note that, in this embodiment, an indium tin oxide (ITO) film or atransparent conductive film in which indium oxide is mixed with zincoxide (ZnO) of 2 to 20% is used as the transparent electrode.

Further, the transparent electrode 367 is formed so as to contact andoverlap the drain wiring 365, thereby having electrical connection witha drain region of an EL driver TFT.

Next, as shown in FIG. 17A, an insulating film containing silicon (asilicon oxide film in this embodiment) is formed with a thickness of 500nm, and an opening portion is formed at the position corresponding tothe transparent electrode 367 to thereby form a third interlayerinsulating film 368 functioning as a bank. In forming the openingportion, sidewalls with a tapered shape may easily be formed by usingthe wet etching method. If the sidewalls of the opening portion are notsufficiently gentle, the deterioration of the EL layer caused by a stepbecomes a marked problem. Thus, attention is required.

Note that, in this embodiment, the silicon oxide film is used as thethird interlayer insulating film, but depending on the situation, anorganic resin film made of polyimide, polyamide, acrylic, or BCB(benzocyclobutene) may also be used.

Subsequently, as shown in FIG. 17A, an EL layer 369 is formed by anevaporation method, and further, a cathode (MgAg electrode) 370 and aprotective electrode 371 are formed by the evaporation method. At thistime, before the formation of the EL layer 369 and the cathode 370, itis desirable that the transparent electrode 367 is subjected to a heattreatment to completely remove moisture. Note that the MgAg electrode isused as the cathode of the EL element in this embodiment, but otherknown materials may also be used.

Note that a known material may be used for the EL layer 369. In thisembodiment, the EL layer adopts a two-layer structure constituted of ahole transporting layer and a light emitting layer. However, there maybe the case where a hole transporting layer, a hole injecting layer, anelectron injecting layer or an electron transporting layer is provided.Various examples of the combination have already been reported, and anystructure of those may be used.

In this embodiment, polyphenylene vinylene is formed by the evaporationmethod as the hole transporting layer. Further, as the light emittinglayer, a material in which 1,3,4-oxydiazole derivative PBD of 30 to 40%is distributed in polyvinyl carbazole is formed by the evaporationmethod, and coumarin 6 of approximately 1% is added as a center of greencolor light emission.

Further, the EL layer 369 can be protected from moisture or oxygen bythe protective electrode 371, but a passivation film 372 is preferablyformed. In this embodiment, a silicon nitride film of 300 nm thicknessis provided as the passivation film 372. This passivation film may alsobe formed in succession after the formation of the protective electrode371 without exposure to an atmosphere.

Moreover, the protective electrode 371 is provided to preventdeterioration of the cathode 370, and is typified by a metal filmcontaining aluminum as its main constituent. Of course, other materialsmay also be used. Further, the EL layer 369 and the cathode 370 are veryweak to moisture. Thus, it is preferable that continuous formation isconducted up through the formation of the protective electrode 371without exposure to an atmosphere to protect the EL layer from theoutside air.

Note that it is appropriate that the thickness of the EL layer 369 is 10to 400 nm (typically 60 to 150 nm) and the thickness of the cathode 370is 80 to 200 nm (typically 100 to 150 nm).

Thus, an EL module with the structure shown in FIG. 17A is completed.Note that, in a process of manufacturing an EL module in thisembodiment, a source signal line is formed from Ta and W, which arematerials forming the gate electrode, and a gate signal line is formedfrom Al that is a wiring material forming the source and drainelectrodes, in connection with the circuit structure and the process.However, different materials may also be used.

Further, a driver circuit 506 having an n-channel TFT 501 and ap-channel TFT 502 and a pixel portion 507 having a switching TFT 503, anEL driver TFT 504, and a capacitor 505 can be formed on the samesubstrate.

Note that, in this embodiment, a structure in which the n-channel TFTand the p-channel TFT are used as the switching TFT 503 and the ELdriver TFT 504, respectively, is shown since the outgoing from a lowersurface is adopted in accordance with the structure of the EL element.However, this embodiment is only one preferred embodiment, and thepresent invention is not necessarily limited to this.

The n-channel TFT 501 of the driver circuit 506 has the channel formingregion 381, the low concentration impurity region 329 (GOLD region) thatoverlaps the first conductive layer 324 a constituting a part of thegate electrode, and the high concentration impurity region 333functioning as the source or drain region. The p-channel TFT 502 has achannel forming region 382, the impurity region 353 that does notoverlap the first conductive layer 340 a constituting a part of the gateelectrode, and the impurity region 350 functioning as the source ordrain region.

The switching TFT 503 of the pixel portion 507 has a channel formingportion 383, the low concentration impurity region 344 (LDD region),which does not overlap the first conductive layer 341 a forming the gateelectrode and which is formed outside the gate electrode, and the highconcentration impurity region 336 functioning as the source or drainregion.

The EL driver TFT 504 of the pixel portion 507 has a channel formingregion 384, and the high concentration impurity regions 352 and 355functioning as the source or drain region. Further, the capacitor 505 isformed such that the first conductive layer 326 a and the secondconductive layer 326 b function as one of the electrodes.

Note that, in this embodiment, although a structure in which the cathodeis formed after the EL layer is formed on the pixel electrode (anode), astructure in which the EL layer and the anode are formed on the pixelelectrode (cathode) may be adopted. Incidentally, in this case,different from the outgoing from a lower surface described above, theoutgoing from an upper surface is adopted. Furthermore, at this time, itis desirable that the EL driver TFT 504 is formed of the n-channel TFT.

This embodiment can be implemented by freely combined with the structureof Embodiments 1 to 3.

Embodiment 5

This embodiment describes with reference to FIGS. 13A to 13C a case ofmanufacturing an EL display device using the present invention.

FIG. 13A is a top view of an EL display device in which a sealing memberis used in sealing. FIG. 13B is a sectional view taken along the lineA-A′ in FIG. 13A. FIG. 13C is a sectional view taken along the line B-B′in FIG. 13A.

A pixel portion 4002, a source signal line driving circuit 4003, andfirst and second gate signal line driving circuits 4004 a and 4004 b areformed on a substrate 4001. A seal member 4009 is placed so as tosurround them all on the substrate. A sealing member 4008 is provided onthe pixel portion 4002, the source signal line driving circuit 4003, andthe first and second gate signal line driving circuits 4004 a and 4004b. Accordingly, the pixel portion 4002, the source signal line drivingcircuit 4003, and the first and second gate signal line driving circuits4004 a and 4004 b are sealed in the space defined by the substrate 4001,the seal member 4009, and the sealing member 4008, with a filler 4210filling the space.

The pixel portion 4002, the source signal line driving circuit 4003, andthe first and second gate signal line driving circuits 4004 a and 4004 bon the substrate 4001 each have a plurality of TFTs. FIG. 13B shows asrepresentatives of these TFTs a driving TFT (composed of an n-channelTFT and a p-channel TFT in FIG. 13B) 4201 included in the source signalline driving circuit 4003 and an EL driving TFT 4202 included in thepixel portion 4002. The TFTs 4201 and 4202 are formed on a base film4010.

In this embodiment, the n-channel TFT and the p-channel TFT thatconstitute the driving TFT 4201 are manufactured by a known method, anda p-channel TFT manufactured by a known method is used for the ELdriving TFT 4202. The pixel portion 4002 is provided with a capacitorstorage (not shown) connected to a gate of the EL driving TFT 4202.

Formed on the driving TFT 4201 and the EL driving TFT 4202 is aninterlayer insulating film (planarization film) 4301, on which a pixelelectrode (anode) 4203 is formed to be electrically connected to a drainof the EL driving TFT 4202. The pixel electrode 4203 is formed of atransparent conductive film having a large work function. Examples ofthe usable transparent conductive film material include a compound ofindium oxide and tin oxide, a compound of indium oxide and zinc oxide,zinc oxide alone, tin oxide alone, and indium oxide alone. A transparentconductive film formed of one of these materials and doped with galliummay also be used for the pixel electrode.

An insulating film 4302 is formed on the pixel electrode 4203. Anopening is formed in the insulating film 4302 above the pixel electrode4203. At the opening above the pixel electrode 4203, an EL(electroluminescence) layer 4204 is formed. The EL layer 4204 is formedof a known organic EL material or inorganic EL material. Either lowmolecular weight (monomer) organic EL materials or high molecular weight(polymer) organic EL materials can be used for the EL layer.

The EL layer 4204 is formed by a known evaporation technique orapplication technique. The EL layer may consist only of a light emittinglayer. Alternatively, the EL layer may be a laminate having, in additionto a light emitting layer, a hole injection layer, a hole transportinglayer, an electron transporting layer, and an electron injection layerin any combination.

A cathode 4205 is formed on the EL layer 4204 from a light-shieldingconductive film (typically, a conductive film mainly containingaluminum, copper, or silver, or a laminate consisting of the aboveconductive film and other conductive films). Desirably, moisture andoxygen are removed as much as possible from the interface between thecathode 4205 and the EL layer 4204. Some contrivance is needed for theremoval. For example, the EL layer 4204 is formed in a nitrogen or raregas atmosphere and then the cathode 4205 is successively formed withoutexposing the substrate to moisture and oxygen. This embodiment uses amulti-chamber system (cluster tool system) film formation apparatus toachieve the film formation as described above. The cathode 4205 receivesa given voltage.

An EL element 4303 composed of the pixel electrode (anode) 4203, the ELlayer 4204, and the cathode 4205 is thus formed. A protective film 4209is formed on the insulating film 4302 so as to cover the EL element4303. The protective film 4209 is effective in preventing oxygen andmoisture from entering the EL element 4303.

Denoted by 4005 a is a lead-out wiring line connected to a power supplyline, and is electrically connected to a source region of the EL drivingTFT 4202. The lead-out wiring line 4005 a runs between the seal member4009 and the substrate 4001 and is electrically connected to an FPCwiring line 4301 of an FPC 4006 through an anisotropic conductive film4300.

The sealing member 4008 is formed of a glass material, a metal material(typically a stainless steel material), a ceramic material, or a plasticmaterial (including a plastic film). Examples of the usable plasticmaterial include an FRP (fiberglass-reinforced plastic) plate, a PVF(polyvinyl fluoride) film, a Mylar film, a polyester film, and anacrylic resin film. A sheet obtained by sandwiching an aluminum foilbetween PVF films or Mylar films may also be used.

However, if light emitted from the EL element travels toward thecovering member side, the covering member has to be transparent. In thiscase, a transparent material such as a glass plate, a plastic plate, apolyester film, or an acrylic film is used.

The filler 4210 may be inert gas such as nitrogen and argon, or aUV-curable resin or a thermally curable resin. Examples of the usableresin include PVC (polyvinyl chloride), acrylic, polyimide, an epoxyresin, a silicone resin, PVB (polyvinyl butylal), and EVA (ethylenevinyl acetate). In this embodiment, nitrogen is used as the filler.

In order to expose the filler 4210 to a hygroscopic substance(preferably, barium oxide) or a substance capable of adsorbing oxygen, ahygroscopic substance 4207, or a substance 4207 capable of adsorbingoxygen, is placed in a concave portion 4007 formed on a surface of thesealing member 4008 on the substrate 4001 side. The hygroscopicsubstance 4207, or a substance 4207 capable of adsorbing oxygen, is helddown to the concave portion 4007 by a concave portion covering member4208 to prevent hygroscopic substance 4207, or a substance 4207 capableof adsorbing oxygen, from scattering. The concave portion coveringmember 4208 is a dense mesh and allows air and moisture to pass but notthe hygroscopic substance 4207, or a substance 4207 capable of adsorbingoxygen. The hygroscopic substance 4207, or a substance 4207 capable ofadsorbing oxygen, can prevent degradation of the EL element 4303.

As shown in FIG. 13C, a conductive film 4203 a is formed to be broughtinto contact with the top face of the lead-out wiring line 4005 a at thesame time the pixel electrode 4203 is formed.

The anisotropic film 4300 has a conductive filler 4300 a. The conductivefiller 4300 a electrically connects the conductive film 4203 a on thesubstrate 4001 to the FPC wiring line 4301 on the FPC 4006 upon athermal press fitting of the substrate 4001 and the FPC 4006.

This embodiment can be combined with any of Embodiments 1 through 4.

Embodiment 6

In this embodiment, an external light emitting quantum efficiency can beremarkably improved by using an EL material by which phosphorescencefrom a triplet exciton can be employed for emitting a light. As aresult, the power consumption of the EL element can be reduced, thelifetime of the EL element can be elongated and the weight of the ELelement can be lightened.

The following is a report where the external light emitting quantumefficiency is improved by using the triplet exciton (T. Tsutsui, C.Adachi, S. Saito, Photochemical processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437).

The molecular formula of an EL material (coumarin pigment) reported bythe above article is represented as follows.

(M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.Thompson, S. R. Forrest, Nature 395 (1998) p.151)

The molecular formula of an EL material (Pt complex) reported by theabove article is represented as follows.

(M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R. Forrest,Appl. Phys. Lett., 75 (1999) p.4.)

(T. Tsutsui, M. -J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tsuji,Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn, Appl. Phys., 38 (12B) (1999)L1502)

The molecular formula of an EL material (Ir complex) reported by theabove article is represented as follows.

As described above, if phosphorescence from a triplet exciton can be putto practical use, it can realize the external light emitting quantumefficiency three to four times as high as that in the case of usingfluorescence from a singlet exciton in principle.

The structure according to this embodiment can be freely implemented incombination of any structures of the Embodiments 1 to 5.

Embodiment 7

This embodiment will be described on electronic devices incorporated adisplay device as a display medium, which is formed by using the presentinvention.

As these electronic devices, there can be enumerated a video camera, adigital camera, a head-mountable display, a game machine, a carnavigation, a personal computer, and a mobile information terminal(e.g., a mobile computer, a mobile telephone or an electronic book), asshown in FIGS. 18A to 18E.

FIG. 18A shows a personal computer including a body 2001, a casing 2002,a display portion 2003 and a keyboard 2004. The display device of thepresent invention can be used as the display portion 2003 of thepersonal computer.

FIG. 18B shows a video camera including a body 2101, a display portion2102, a voice input unit 2103, manipulation switches 2104, a battery2105 and an image receiving unit 2106. The display device of the presentinvention can be used as the display portion 2102 of the video camera.

FIG. 18C shows one portion (i.e., a right-hand side) of a head-mounteddisplay including a body 2301, a signal cable 2302, a head fixing band2303, a display unit 2304, an optical system 2305 and a display portion2306. The display device of the present invention can be used thedisplay portion 2306 of the head-mounted display.

FIG. 18D shows an image reproducing device (e.g., a DVD reproducingdevice) provided with a recording medium. The image reproducing deviceincludes a body 2401, a recording medium (CD, LD or DVD and the like)2402, manipulation switches 2403 and display units (a) 2404 and (b)2405. The display portion 2404 (a) displays an image information and thedisplay portion (b) 2405 displays character information. The displaydevice using a driving method of the present invention can be used thedisplay portions (a) 2404 and (b) 2405. Here, this device is enabled toCD reproduction device and the game device as an image reproductiondevice incorporating the recording medium.

FIG. 18E shows a mobile computer including a body 2501, a camera portion2502, a image receiving unit 2503, an operation switch 2504 and adisplay portion 2505. The display device of the present invention can beused as the display portion 2505 of the mobile computer.

As has been described hereinbefore, the invention can have an extremelywide range of applications and can be applied to electronic devices ofany fields. On the other hand, the electronic device of this embodimentcan be realized by using a construction of any of the combinations ofEmbodiments 1 to 6.

Conventionally, it has been difficult to obtain accurate gray scalesbecause a current inputted to an EL element does not correspond to thesignal electric potential of a video signal linearly.

With the above structure, the present invention can cause an EL elementto emit light at a luminance linearly corresponding to the signalelectric potential of a video signal. The present invention thusprovides a display device that is easy to obtain accurate gray scaledisplay.

What is claimed is:
 1. A display device comprising: an EL element havinga first electrode, a second electrode, and an EL layer that isinterposed between the first electrode and the second electrode; a firstTFT; a second TFT having the same polarity as the first TFT, the secondTFT including a gate electrode and a drain region that are connected toeach other; and a power supply line connected to the first electrode ofthe EL element through the first TFT, wherein the display device furthercomprises: means for setting a current linearly corresponding to theelectric potential difference between the electric potential of thepower supply line and the electric potential of a video signal as adrain current of the second TFT; and means for equalizing the gatevoltage of the first TFT with the gate voltage of the second TFT.
 2. Adisplay device comprising: an EL element having a first electrode, asecond electrode, and an EL layer that is interposed between the firstelectrode and the second electrode; a first TFT; a second TFT having thesame polarity and the same threshold voltage as the first TFT, thesecond TFT including a gate electrode and a drain region that areconnected to each other; and a power supply line connected to the firstelectrode of the EL element through the first TFT, wherein the displaydevice further comprises: means for setting a current linearlycorresponding to the electric potential difference between the electricpotential of the power supply line and the electric potential of a videosignal as a drain current of the second TFT; and means for equalizingthe gate voltage of the first TFT with the gate voltage of the secondTFT.
 3. A display device comprising: an EL element having a firstelectrode, a second electrode, and an EL layer that is interposedbetween the first electrode and the second electrode; a first TFT; asignal line; a switching element for selecting the connection betweenthe signal line and a gate electrode of the first TFT; a second TFThaving the same polarity as the first TFT, the second TFT including agate electrode and a drain region that are connected to each other; anda power supply line connected to the first electrode of the EL elementthrough the first TFT, wherein the display device further comprises:means for setting a current linearly corresponding to the electricpotential difference between the electric potential of the power supplyline and the electric potential of a video signal as a drain current ofthe second TFT; means for equalizing the electric potential of a sourceregion of the first TFT with the electric potential of a source regionof the second TFT; and means for equalizing the electric potential ofthe signal line with the electric potential of the gate electrode of thesecond TFT.
 4. A display device comprising: an EL element having a firstelectrode, a second electrode, and an EL layer that is interposedbetween the first electrode and the second electrode; a first TFT; asecond TFT; a signal line connected to a gate electrode of the first TFTthrough the second TFT; a third TFT having the same polarity as thefirst TFT, the third TFT including a gate electrode and a drain regionthat are connected to each other; and a power supply line connected tothe first electrode of the EL element through the first TFT, wherein thedisplay device further comprises: means for setting a current linearlycorresponding to the electric potential difference between the electricpotential of the power supply line and the electric potential of a videosignal as a drain current of the third TFT; means for equalizing theelectric potential of a source region of the first TFT with the electricpotential of a source region of the third TFT; and means for equalizingthe electric potential of the signal line with the electric potential ofthe gate electrode of the third TFT.
 5. A display device comprising: anEL element having a first electrode, a second electrode, and an EL layerthat is interposed between the first electrode and the second electrode;a first TFT having a drain region that is connected to the firstelectrode of the EL element; a second TFT having the same polarity asthe first TFT, the second TFT including a gate electrode and a drainregion that are connected to each other; a power supply line connectedto a source region of the first TFT; and an operation amplifier having anon-inversion input terminal, an inversion input terminal, and an outputterminal, the non-inversion input terminal being kept at an electricpotential equal to the electric potential of the power supply line, theinversion input terminal being connected to a source region of thesecond TFT, and the output terminal being connected to the drain regionof the second TFT, wherein the display device further comprises: meansfor setting a current linearly corresponding to the electric potentialdifference between the electric potential of the power supply line andthe electric potential of a video signal as a drain current of thesecond TFT; and means for equalizing the electric potential of a gateelectrode of the first TFT with the electric potential of the gateelectrode of the second TFT.
 6. A display device comprising: an ELelement having a first electrode, a second electrode, and an EL layerthat is interposed between the first electrode and the second electrode;a first TFT having a drain region that is connected to the firstelectrode of the EL element; a signal line; a switching element forselecting the connection between the signal line and a gate electrode ofthe first TFT; a second TFT having the same polarity as the first TFT,the second TFT including a gate electrode and a drain region that areconnected to each other; a power supply line connected to a sourceregion of the first TFT; and an operation amplifier having anon-inversion input terminal, an inversion input terminal, and an outputterminal, the non-inversion input terminal being kept at an electricpotential equal to the electric potential of the power supply line, theinversion input terminal being connected to a source region of thesecond TFT, and the output terminal being connected to the drain regionof the second TFT, wherein the display device further comprises: meansfor setting a current linearly corresponding to the electric potentialdifference between the electric potential of the power supply line andthe electric potential of a video signal as a drain current of thesecond TFT; and means for equalizing the electric potential of thesignal line with the electric potential of the gate electrode of thesecond TFT.
 7. A display device comprising: an EL element having a firstelectrode, a second electrode, and an EL layer that is interposedbetween the first electrode and the second electrode; a first TFT havinga drain region that is connected to the first electrode of the ELelement; a second TFT; a signal line connected to a gate electrode ofthe first TFT through the second TFT; a third TFT having the samepolarity as the first TFT, the third TFT including a gate electrode anda drain region that are connected to each other; and a power supply lineconnected to a source region of the first TFT; and an operationamplifier having a non-inversion input terminal, an inversion inputterminal, and an output terminal, the non-inversion input terminal beingkept at an electric potential equal to the electric potential of thepower supply line, the inversion input terminal being connected to asource region of the third TFT, and the output terminal being connectedto the drain region of the third TFT, wherein the display device furthercomprises: means for setting a current linearly corresponding to theelectric potential difference between the electric potential of thepower supply line and the electric potential of a video signal as adrain current of the third TFT; and means for equalizing the electricpotential of the signal line with the electric potential of the gateelectrode of the third TFT.
 8. A display device comprising: an ELelement having a first electrode, a second electrode, and an EL layerthat is interposed between the first electrode and the second electrode;a first TFT having a drain region that is connected to the firstelectrode of the EL element; a second TFT having the same polarity asthe first TFT, the second TFT including a gate electrode and a drainregion that are connected to each other; a power supply line connectedto a source region of the first TFT; an operation amplifier having anon-inversion input terminal, an inversion input terminal, and an outputterminal, the non-inversion input terminal being kept at an electricpotential equal to the electric potential of the power supply line, theinversion input terminal being connected to a source region of thesecond TFT, and the output terminal being connected to the drain regionof the second TFT; and a resistor having a first terminal to which theelectric potential of a video signal is inputted and a second terminalconnected to the inversion input terminal of the operation amplifier,wherein the display device further comprises means for equalizing theelectric potential of a gate electrode of the first TFT with theelectric potential of the gate electrode of the second TFT.
 9. A displaydevice according to any one of claims 3, 5, 6 and 8, wherein the firstTFT and the second TFT have substantially same threshold voltage.
 10. Adisplay device according to any one of claims 4 and 7, wherein the firstTFT and the third TFT have substantially same threshold voltage.
 11. Adevice according to any one of claims 1 to 8, wherein the display deviceis incorporated into an electronic equipment selected from the groupconsisting of a video camera, a digital camera, a head-mountabledisplay, a game machine, a car navigation, a personal computer, and amobile information terminal.
 12. A method of driving a display devicecomprising the steps of: causing a drain current of a first TFT to flowbetween a first electrode and a second electrode of an EL element, theEL element consisting of the first electrode, the second electrode, andan EL layer that is interposed between the first electrode and thesecond electrode; and equalizing the electric potential of a sourceregion of the first TFT with the electric potential of a power supplyline, wherein: a current linearly corresponding to the electricpotential difference between the electric potential of the power supplyline and the electric potential of a video signal is set as a draincurrent of a second TFT, the second TFT having the same polarity as thefirst TFT; the second TFT has a gate electrode and a drain region thathave the same electric potential; and the electric potential of thepower supply line shifts by an amount corresponding to the gate voltageof the second TFT and the electric potential including the shift isinputted to a gate electrode of the first TFT.
 13. A method of driving adisplay device comprising the steps of: causing a drain current of afirst TFT to flow between a first electrode and a second electrode of anEL element, the EL element consisting of the first electrode, the secondelectrode, and an EL layer that is interposed between the firstelectrode and the second electrode; and equalizing the electricpotential of a source region of the first TFT with the electricpotential of a power supply line, wherein: a current linearlycorresponding to the electric potential difference between the electricpotential of the power supply line and the electric potential of a videosignal is set as a drain current of a second TFT, the second TFT havingthe same polarity and the same threshold voltage as the first TFT; thesecond TFT has a gate electrode and a drain region that have the sameelectric potential; and the electric potential of the power supply lineshifts by an amount corresponding to the gate voltage of the second TFTand the electric potential including the shift is inputted to a gateelectrode of the first TFT.
 14. A method of driving a display devicecomprising the steps of: causing a drain current of a first TFT to flowbetween a first electrode and a second electrode of an EL element, theEL element consisting of the first electrode, the second electrode, andan EL layer that is interposed between the first electrode and thesecond electrode; and equalizing the electric potential of a sourceregion of the first TFT with the electric potential of a power supplyline, wherein: a current linearly corresponding to the electricpotential difference between the electric potential of the power supplyline and the electric potential of a video signal is set as a draincurrent of a second TFT, the second TFT having the same polarity as thefirst TFT; the voltage between an inversion input terminal and an outputterminal of an operation amplifier is the same as the voltage between asource region and a drain region of the second TFT, the operationamplifier having a non-inversion input terminal that is kept to anelectric potential equal to the electric potential of the power supplyline; a gate electrode of the second TFT and the drain region thereofhave the same electric potential; and the electric potential of theoutput terminal of the operation amplifier is inputted to a gateelectrode of the first TFT.
 15. A method of driving a display devicecomprising the steps of: causing a drain current of a first TFT to flowbetween a first electrode and a second electrode of an EL element, theEL element consisting of the first electrode, the second electrode, andan EL layer that is interposed between the first electrode and thesecond electrode; and equalizing the electric potential of a sourceregion of the first TFT with the electric potential of a power supplyline, wherein: a current flowing through a resistor is set as a draincurrent of a second TFT that has the same polarity as the first TFT, theresistor having a first terminal to which the electric potential of avideo signal is inputted and a second terminal connected to an inversioninput terminal of an operation amplifier; the voltage between theinversion input terminal of the operation amplifier and an outputterminal thereof is the same as the voltage between a source region anda drain region of the second TFT, the operation amplifier having anon-inversion input terminal that is kept to an electric potential equalto the electric potential of the power supply line; a gate electrode ofthe second TFT and the drain region thereof have the same electricpotential; and the electric potential of the output terminal of theoperation amplifier is inputted to a gate electrode of the first TFT.16. A method of driving a display device according to any one of claims14 and 15, wherein the first TFT and the second TFT have the samethreshold voltage.
 17. A method of driving a display device according toany one of claims 12 to 16, wherein the first TFT is a p-channel TFT,and wherein the electric potential of the output terminal of theoperation amplifier is kept lower than the electric potential of thenon-inversion input terminal of the operation amplifier.
 18. A method ofdriving a display device according to any one of claims 12 to 16,wherein the first TFT is an n-channel TFT, and wherein the electricpotential of the output terminal of the operation amplifier is kepthigher than the electric potential of the non-inversion input terminalof the operation amplifier.
 19. A method of driving a display deviceaccording to any one of claims 12 to 16, wherein the display device isincorporated into an electronic equipment selected from the groupconsisting of a video camera, a digital camera, a head-mountabledisplay, a game machine, a car navigation, a personal computer, and amobile information terminal.